Method for the continuous recovery of carbon dioxide from acidified seawater

ABSTRACT

A method for recovering carbon dioxide from acidified seawater using a membrane contactor and passing seawater with a pH less than or equal to 6 over the outside of a hollow fiber membrane tube while applying vacuum or a hydrogen sweep gas to the inside of the hollow fiber membrane tube, wherein up to 92% of the re-equilibrated [CO 2 ] T  is removed from the natural seawater.

PRIORITY CLAIM

This Application claims priority from U.S. Provisional Application No.61/522,743 filed on Aug. 12, 2011 by Heather D Willauer et al., entitled“METHOD FOR THE CONTINUOUS RECOVERY OF CARBON DIOXIDE FROM ACIDIFIEDSEAWATER,” the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to CO₂ extraction and, morespecifically, to recovery of CO₂ from acidified natural seawater.

2. Description of the Prior Art

The total carbon content of the world's oceans is roughly 38,000 GtC(gigaton of carbon). Over 95% of this carbon is in the form of dissolvedbicarbonate ion (HCO₃ ⁻). This ion along with carbonate is responsiblefor buffering and maintaining the ocean's pH, which is relativelyconstant below the first 100 meters. This dissolved bicarbonate andcarbonate is essentially bound CO₂, and as shown in equation (1), thesum of these species along with gaseous CO₂ represents the total carbondioxide concentration [CO₂]_(T) of seawater, defined as:[CO₂]_(T)=[CO₂(g)]+[HCO₃ ⁻]+[CO₃ ²⁻]  (1)

At a typical ocean pH of 7.8, [CO₂]_(T) is about 2000 μmoles/kg near thesurface and 2400 μmoles at depths below 300 meters. Takahashi et al.,“The Alkalinity and Total Carbon Dioxide Concentration in the WorldOceans,” Carbon Cycle Modeling, John Wiley and Sons, New-York, 271-286(1981); Takahashi et al., “Carbonate Chemistry of the Surface of theWaters of the World Oceans,” Isotope Marine Chemistry, Uchida Rokakuho,Tokyo, Japan, 291-326 (1980), the entire contents of each areincorporated herein by reference. This equates to approximately 100 mg/Lof [CO₂]_(T) of which 2 to 3% is CO₂ (g), 1% is dissolved carbonate, andthe remainder is dissolved bicarbonate. The acidification of naturalseawater offers an indirect approach to recovery of CO₂ from itsbicarbonate and carbonate form from the equilibrium conditions of CO₂ inseawater shown in equation (2). Johnson et al., “Coulometric TCO₂Analyses for Marine Studies: An Introduction,” Marine Chem., 16, 61(1985), the entire contents of which is incorporated herein byreference. Johnson et al. demonstrated that carbonate and bicarbonatere-equilibrate to CO₂ gas at seawater pH less than or equal to 6. Thismethod has been the basis for standard quantitative ocean CO₂measurements for over 25 years.HCO₃ ⁻+H⁺⇄H₂CO₃⇄H₂O+CO₂(g)↑  (2)

An electrochemical method to acidify seawater and recover [CO₂]_(T)simultaneously with hydrogen gas from alkaline water sources such asseawater has been developed at the Naval Research Laboratory. (U.S.patent application Ser. No. 12/958,963 filed on Nov. 17, 2011 by FeliceDiMascio, the entire contents of which are incorporated herein byreference.) FIG. 1 illustrates the percent removal of [CO₂]_(T) plottedas a function of pH for effluent seawater samples taken from theelectrochemical acidification cell during a 40 minute constant polaritycycle. This figure indicates that approximately 50% of the [CO₂]_(T) isspontaneously degassed from the effluent natural seawater at pH ≦4. Tocomplement this technology, the Naval Research Laboratory has developedthe present invention to enhance re-equilibrated [CO₂]_(T) (see equation2) recovery up to 92% from seawater at pH of ≦4.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for recovering re-equilibrated[CO₂]_(T) from acidified natural seawater using a membrane contactor andpassing seawater with a pH ≦6 over the outside of the hollow fibermembrane tube while applying vacuum or a hydrogen sweep gas to theinside of the membrane tube, wherein up to 92% of the re-equilibrated[CO₂]_(T) (see equation 2) is removed from the natural seawater.

The method of the present invention requires only vacuum or a sweep gasto recover low concentrations of CO₂ from acidified seawater (˜50 mg/L).In the case of vacuum, CO₂ can be concentrated in proportions needed forfeedstock. In the case of hydrogen as a sweep gas, no additional energypenalty is required to recover the CO₂. The hydrogen/CO₂ mixtureproduced in the acidification of the seawater can be used directly as afeedstock.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of re-equilibrated [CO₂]_(T) spontaneous degassing as afunction of pH for effluent natural seawater samples taken from theelectrochemical acidification cell during a 40 minute constant polaritycycle at 20 amps and seawater flow rate of 0.5 gal/minute.

FIG. 2 is a schematic diagram of a polyethylene hollow fiber membranecontactor.

FIG. 3 is a plot of re-equilibrated [CO₂]_(T) removal as a function ofpH for effluent natural seawater samples taken from the electrochemicalacidification cell during two consecutive 45 minute constant polaritycycles at 20 amps after contact with hollow fiber membrane contactor ata seawater flow rate of 0.5 gal/minute and vacuum conditions ˜30 inchesof Hg.

FIG. 4 is a plot of re-equilibrated [CO₂]_(T) removal as a function ofpH for effluent natural seawater samples taken from the electrochemicalacidification cell during a 30 minute constant polarity cycles at 30amps after contact with hollow fiber membrane contactor at seawater flowrate of 0.5 gal/minute and hydrogen sweep gas flow rates of ˜1500mL/min.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to the continuous recovery ofre-equilibrated [CO₂]_(T) from acidified natural seawater. A hollowfiber membrane contactor is used to increase the seawater surface area.A vacuum is applied to the inside of hollow fiber membrane tubes in thecontactor as effluent seawater is passed over the outside of the hollowfiber membrane tubes. Alternatively, the vacuum can be replaced with ahydrogen sweep gas on the inside of the hollow fiber membrane tubes.Using either method, up to 92% of the re-equilibrated [CO₂]_(T) inseawater can be removed.

The present invention provides an efficient method to recover up to 92%of the re-equilibrated carbon dioxide from acidified natural seawater inflowing systems. At seawater pH below or equal to 6 it is not readilyobvious that only a fraction (up to 50 mg/L or 50% recovery) ofre-equilibrated [CO₂]_(T) in acidified natural seawater willspontaneously degas in flowing systems. The solubility ofre-equilibrated [CO₂]_(T) in natural seawater requires additionalmechanisms that increase seawater surface area so that up to 92% of there-equilibrated [CO₂]_(T) can be recovered from the acidified naturalseawater in flowing systems.

The re-equilibration of [CO₂]_(T) shown in equation 2 from dissolvedcarbonate and bicarbonate in natural seawater and its solubility isdependent on carbonate and bicarbonate speciation, seawater temperature,salinity, pressure, and ion association. Warmer seawater temperaturesand higher salt concentrations change the bicarbonate and carbonateequilibrium with the dissolved CO₂ in the form of carbonic acid, and theoverall CO₂ solubility in the solution.

It is known that CO₂ gas solubility is reduced in aqueous salt solutionscompared to deionized water solutions. (Follows et al., “On the solutionof the carbonate chemistry system in ocean biogeochemistry models,”Ocean Modelling, 12, 290-301 (2006); Al.-Anezi et al., “Scale formationin desalination plants: effect of carbon dioxide solubility,”Desalination, 204, 385-402 (2007); Al.-Anezi et al., “Parametersaffecting the solubility of carbon dioxide in seawater at the conditionsencountered in MSF desalination plants,” Desalination, 222, 548-571(2008); Willauer et al., “Development of an ElectrochemicalAcidification Cell for Recovery of CO2 and H2 from Seawater II.Evaluation of the Cell by Natural Seawater,” Industrial & Engineer Chem.Res., DOI: 10.1021/ie301006y (2 Aug. 2012), the entire contents of eachare incorporated herein by reference.) As saltwater salinity increasesand seawater temperatures increase, LeChatlier's Principle dictatesequilibrium shift to the left (Equation 2). The result is a decrease inCO₂ solubility that leads to the greater ease of degassing of CO₂ fromthe seawater. Changes in CO₂ solubility result in changes in vacuumrequirements needed to recover up to 92% of [CO₂]_(T) for the enclosedinvention.

FIG. 2 is a diagram of a Liqui-Cel® polyethylene hollow fiber membranecontactor. Gas permeable membranes are available commercially for theremoval or addition of gases from liquids. Most of these applicationsare near atmospheric pressure and include water purification, bloodoxygenation and artificial lung devices. However, some are operated athigher pressures, such as beverage carbonation. These membranes work onthe principle of dissolved gases such as carbon dioxide diffusing acrossthe membrane through the pores as a function of differential partial gaspressures.

A Liqui-Cel® polyethylene hollow fiber membrane contactor was used as amethod to increase natural seawater surface area and thus increasere-equilibrated [CO₂]_(T) recovery from natural seawater at pH ≦6. Up to30 inches of Hg vacuum was applied by a vacuum pump to the inside of thehollow fiber membrane tubes in the contactor as effluent naturalseawater passed over the outside of the hollow fiber membrane tubes at aflow rate of 0.5 gallons/minute. FIG. 3 shows the percent removal ofre-equilibrated [CO₂]_(T) plotted as a function of effluent seawater pHfor two consecutive 45 minute polarity cycles at 20 amps of appliedcurrent. From FIG. 3, it is clear up to 92% of the re-equilibrated[CO₂]_(T) was removed from the effluent seawater at pH ≦4 at high vacuumconditions (˜30 inches of Hg). As the effluent seawater pH was loweredfurther to 2.6, no measureable increase in CO₂ removal was observed.

Since the removal of gases is a function of differential partial gaspressures and hydrogen is being produced at the cathode of theelectrochemical cell at a 3 to 1 ratio of H₂ to CO₂, hydrogen was usedas a sweep gas in replacement of the vacuum on the inside of the hollowfiber membrane tubes. Using hydrogen as the sweep gas enables the gasesto be pre-mixed before being used as the feedstock in hydrogenation tovalue added hydrocarbons. This method also saves additional energiesthat would be expended by using a vacuum pump.

FIG. 4 shows that when hydrogen is used as the sweep gas at ˜1500mL/min, re-equilibrated [CO₂]_(T) recovery is up to 92%. Table 1indicates that as hydrogen concentration is decreased from ˜1500 mL/minto 360 mL/min, re-equilibrated [CO₂]_(T) recovery is reduced from up to92% to 69%. At a hydrogen flow rate of 160 mL/min there is no measurableincrease in re-equilibrated [CO₂]_(T) recovery above the 49% removed byspontaneous degassing.

TABLE 1 Re-equilibrated [CO₂]_(T) removal as a function of hydrogensweep gas flow rates for effluent natural seawater samples at pH ≦3.Hydrogen Gas Sweep Sample (mL/min) CO₂ Removal (%) 1 ~1500 92 2 ~360 693 ~160 49

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method for the recovery of re-equilibrated[CO₂]_(T) from acidified natural seawater, comprising: passing naturalseawater with a pH less than or equal to 6 over the outside of a hollowfiber membrane tube in a membrane contactor; and simultaneously applyingvacuum to the inside of the hollow fiber membrane tube; wherein 69% to92% of the re-equilibrated [CO₂]_(T) is removed from the naturalseawater.
 2. The method of claim 1, wherein the vacuum applied isgreater than zero to 30 inches of Hg.
 3. The method of claim 1, whereinthe seawater has a pH less than or equal to
 4. 4. A method for therecovery of re-equilibrated [CO₂]_(T) from acidified natural seawater,comprising: passing seawater with a pH less than or equal to 6 over theoutside of a hollow fiber membrane tube in a membrane contactor; andsimultaneously using hydrogen as a sweep gas on the inside of the hollowfiber membrane tube at a flow rate of 360 mL/min or higher; wherein 69to 92% of the carbon dioxide is removed from the natural seawater. 5.The method of claim 4, wherein the hydrogen sweep gas has a flow rate of1500 mL/min.
 6. The method of claim 4, wherein the seawater has a pHless than or equal to 4.